Interval timing apparatus and method



Oct. 20, 1959 R. GODBARSEN, JR 2,909,666

INTERVAL TIMING APPARATUS AND METHOD Filed July 14. 1953 5 Sheets-Sheet l uvs TIME IN TERMS OF RAY PULSATION CYCLES AT I20 CYCLES PER SEC.

CYCLES u R Wu 0 5 T R 7 N. O 2 ER 0 0 WJ "H In A 5 F E 2 6 A a o. B o w G T R 5 E 0 w v.... 0 RB S D N 0 C E S 5 2 O O 5 Q 0 Q 0 BIAS VOLTAGE PER CENT OF MAXIMUM BIAS CONDENSER CHARGING RANGE-PERCENT.

Oct. 20, 1959 R. GODBARSEN, JR

INTERVAL TIMING APPARATUS AND METHOD Filed July 14, 1953 5 Sheets-Sheet 2 I??? 90 a x l 75 80 P j E J I Pb THEORETICAL cmmsme RANGE 70 OF INTEGRATING couosussa a PT REQUIRED FOR ADVANCED 1,2 THYRATRON OPERATION TO 60 a conmzc'r FOR counc'ron LAG 3,) OF 0.0m sacouos P3 (b) OF 0.009 szcouos 50 2/ FIG 3 30 a 0 2 3 4 s s 7 a 9 w I2 l4 POSITIVE HALF CYCLE RAYIPULSATIONS- TIME IN TERMS OF RAY PULSATION S AT I CYCLES PER- SEC.

CYCLES SECONDS FIG.4

THEORETICAL OPERATION OF. TNYRATRON UNDER THE CONTROL OF AN INTEGRATING CONDENSER TO COMPENSATE FOR A OONTAOTOR LAG OF THE ORDER OF O-OOSSECONDS mvemoaa ROBERT GODBARSEN J R.

' ATTORNEY Oct. 20, 1959 R. GODBARSEN, JR

INTERVAL TIMING APPARATUS AND METHOD s w t G F m 5 m m; m N M O S RG D .t flDE Q N ANWOWF 0 h R 0 C S E N A 8 RT 2 P F N 6 5 00 OF- R $60 0 LTLN R .R MAMMAO 5 WW? 1 5 PO.- AWCGFT E c m 5v Y 60 A I 00 R P 3 CYCLES 0.025 SECONDS TIME IN TERMS OF RAY PULSATIONS AT FIG.5

RAY EXCESS ERROR Filed July 14, 1953 I 00 0O w34ww7 m 25 2:55 6 EH. SE wo 5o msm ON 4 OFF- ON 5 OFF PULSATIONS ON 2 OFF- ON 3 OFF TIME IN TERMS OF RAY on OFF mwwm 23232 2 m0 PZmQ mwm o s w INVENTORl- ROBERT GODBARSEN, JR. BY-

ATTORNEY Oct. 20, 1959 R. GODBARSEN, JR 2,909,666

INTERVAL TIMING APPARATUS AND METHOD 5 Sheets-Sheet 4 Filed July 14, 1953 INVENTOR'.-

R- J N 1 R N O m A A m a G T P R E B m Oct. 20, 1959 R. GODBARSEN, JR 2,

INTERVAL TIMING APPARATUS AND METHOD Filed July 14, 1953 5 Sheets-Sheet 5 H6 8 VOLTAGE on CONDUCTOR a:

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H6 9 smo- CATHODE VOLTAGE VALVE v,

VOLTS FIG IO GRID- CATHODE VOLTAGE VALVE v 3v. o.c. F|G GRID-CATHODE VOLTAGE-VALVE v 0 VOLTS of. V H

FIG '2 smo- CATHODE VOLTAGE "VALVE v as 78 a:

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ATTORNEY United States Patent INTERVAL TIMING APPARATUS AND METHOD Robert Godbarsen, Jr., Milwaukee, Wis., assignor to General Electric Company, a corporation of New York Application July 14, 1953, Serial No. 367,937

8 Claims. (Cl. 250-95) The present invention relates in general to the measurement of elapsed time intervals, and has more particular reference to the measurement of exposure intervals, the

invention pertaining specifically to the provision of means for timing relatively short exposure intervals of the sort frequently employed in X-ray photography.

Timing apparatus of the character mentioned may comprise means operable to measure any elapsed time interval, within the range of the equipnient'and controllingly associated with translation circuitry for the actuation of relay switch means at the beginning and end of the measured interval, such relay switch means being connected with apparatus such as ray generating equip ment to be operated during the measured time interval. An inherent characteristic of relay switches is that the same do not operate instantly. Because of inertia forces, there is always a time delay or lag interval between the instant at which a thyratron fires and the moment of efiective response of the thyratron controlled relay switch means in disabling the associated ray generating apparatus, to thus terminate the exposure interval. While this time delay or operational lag may be relatively short, that is to say, of the order of 0.005 to 0.01second, per switch, or 0.01 to 0.02 second where a pair of successively operating switch devices are involved, the delay, nevertheless, may represent a substantial and undesirable prolongation of an exposure interval, particularly where the exposure interval being measured is short. Where the exposure interval is of the order of one-fifth second, a prolongation of the interval by as much as 0.02 second will represent a 10 percent increase in the duration of the exposure interval. The prolongation, by 0.02 second, of

an exposure interval of one-fiftieth second in duration, however, represents a 100 percent increase in the duration of the exposure interval.

An important object of the present invention is to provide an interval measuring or timing system of the char- .acter described having means for anticipating the end of .a timed interval and for starting operable equipment, such as thyratron actuated relay switch means, in operation, sufliciently in advance of the end of the interval being timed, to oitset the lag or delay in the eifective operation of such operable equipment, whereby to accomplish a desired control function precisely at the end of the interval being timed.

by a thyratron valve under the control of an integrating condenser, by applying a varying negative bias in the system during the interval being timed, in order to cause the thyratron to fire sufficiently in advance of the end of the interval being timed to compensate for the operational lag of the switch means; a further object being to exponentially vary said compensating bias.

Another important object is to compensate for the operational lag time of mechanical elements in an interval measuring system embodying a thyratron valve for Patented Oct. 20,, 1959 actuating said elements at the end of the measured interval, which consists in determining empirically or otherwise the operational lag interval of said elements and applying a variable negative bias in the system to cause the valve to operate, in advance of the end of the interval being measured, by an interval equal to the operational lag interval of said elements, regardless of the duration of the interval being measured.

Another important object is to employ a control circuit embodying resistance and capacity factors for applying a variable negative bias in a system of the character mentioned.

Apparatus for measuring ray exposure intervals by means of a thyratron valve operable under the control of an integrating condenser to de-limit the end of an interval being measured, may be operated by continuously charging the condenser, during the interval being measured,

at a rate equal to the average ray quanta delivered during the interval, such arrangement being satisfactorily accurate where the rays are emitted continuously. It will be seen, however, that where the rays are emitted in pulses, the continuous delivery of charging current to an integrating condenser, during a period being measured, will result in the actuation of the connected thyratron sooner than the same would be actuated if the condenser were charged pulsatingly and precisely in accordance with the pulsations of the emitted rays. 7

Accordingly, it is an important object of the present invention to provide means in a system of the character mentioned for pulsatingly charging an integrating condenser in precise conformity with pulsating rays emitted during an exposure interval being measured.

Another important object is to provide means precisely responsive to the energy quanta of pulsating rays for measuring a ray exposure interval in terms of total ray quanta emitted pulsatingly during the interval and for actuating operating equipment, having inherent operational time lag characteristics, sufficiently in advance of the end of the interval being timed to permit the equipment to effectively complete its operational action precisely coincidentally with the end of the interval being measured.

Another important object is to include, in the integrating system, switch means operable to disable the integrating condenser during intervals between successive ray pulsations when no rays are delivered from the ray source; a still further object being to utilize electronic switching means for thus disabling the integrating condenser.

Another important object is to compensate for the operational time lag of mechanical equipment in an X-ray phototiming system embodying a thyratron valve, controlled by an integrating condenser, for actuating said equipment at the end of an exposure interval, which consists in determining said operational time lag, empirically or otherwise, and then providing for applying an exponentially variable bias on the integrating condenser to cause the same to actuate the thyratron to start the mechanical equipment in operation, in advance of the end of the interval being timed by a period equal to the lag interval of said equipment; a further object being to calibrate the equipment to bias the apparatus in accordance with the time constant of a resistance-capacity timing circuit approximating a selected exponential function, thereby providing the required bias conditions for causing precisely accurate operation of the equipment within a limited interval measuring range; a still further object being to provide equipment of the character mentioned embodying a plurality of resistance-capacity biasing systems, each adapted to bias the system in accordance with a'corresponding time constant to accurately produce required bias conditions within successive limited timing ranges within the over-all timing range of the equipment.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawings discloses a preferred embodiment of the invention.

Referring to the drawings:

Fig. 1 is a diagrammatic view of conventional interval measuring apparatus of the sort embodying a thyratron valve controlled by a ray responsive integrating condenser, the same providing an operational environment for the present invention;

Fig. 2 is a graphical representation of the performance of the integrating condenser in measuring ray quanta at various delivery rates in the conventional system shown in Fig. 1;

Figs. 3, 4, 5 and 6 are graphs illustrating the manner in which the integrating condenser is necessarily controlled, in accordance with the present invention, in order to correct for the operational lag times of operable meanscontrollably associated with the thyratron valve;

Fig. 7 is a diagrammatic view of interval measuring apparatus embodying the present invention;

Figs. 8, 9, 10, 11 and 12 are graphs illustrating the operation of the system shown in Fig. 7; and

Fig. 13 is a diagrammatic showing of a preferred form of light sensitive detector which may be employed in systems embodying the invention.

To illustrate the invention, the drawings show a controllable source of X-rays comprising a conventional X-ray generating tube 11 having an electron emitting cathode 12 and an anode 13 forming an electron target disposed in position to receive the impact of electrons emitted by and at the cathode. The cathode and anode may be connected respectively with conductors 15 and '16 which extend outwardly of the envelope 14, through suitable seals, for the purpose of applying tube energizing power to excite the cathode for electron emission, and to drive cathode emitted electrons at high speed toward the anode under the influence of electron driving potential applied between the anodeand cathode. impingement of electrons upon the cathode facing target surface of the anode 13 constitutes the target surface as an X-ray source from which the generated X-rays may be emitted outwardly of the envelope in the form of an X-ray beam '17.

To operate the tube 11 as an X-ray generator, the anode conductor 16 and one of the cathode conductors 15 may be connected with the secondary winding 18 of a step-up transformer 19, either directly, where the genera tor is to be operated as a self-rectified device, or through a suitable rectifier R if the device is to be operated by full wave rectified alternating current power.

To energize the transformer 19, its primary winding 20 may be connected with the secondary winding 21 of a step-up transformer 23. The primary winding 24 of this step-up transformer may be connected with a suitable power source 25, preferably through a disconnecting switch 26. Means, such as an adjustable connection 2-7 with the secondary winding 21, may be and preferably is provided for varying the potential applied between the anode and cathode. To energize the cathode 1-2 for electron emission, the conductors 15 may be connected with a secondary winding 28 of the transformer 23, preferably through an adjustable connection 29.

In order to start and stop the generation of X-rays, a control switch 30 may be interposed at any convenient location in the anode-cathode power supply system, the switch 30, as shown, being preferably connected in the power supply circuit to the primary winding of the transformer 19. The switch 30 may comprise a normally open relay switch adapted to be closed, by an associated coil or solenoid 31, when and so long as said coil is energized, as from the source 25, i

The X-ray beam 17 may be applied toany useful purpose, including the therapeutic irradiation of the body 33 of a patient, which may be supported for treatment in the path of the beam 17, as on a treatment table 34. The beam 17 may also be usefully employed for the production of X-ray shadow pictures of the body 33 or other object to be pictured, by mounting a sheet of X-ray sensitive film material 35, preferably enclosed in a suitable lighttight cassette 36 in the path of the beam 17 on the side of the body 33 remote from the X-ray source, so that the film material 35 may be exposed to the action of the ray beam after it shall have passed through the body '33, to thus impose, upon the film, an X-ray shadow picture of the body, in accordance with known X-ray photography procedures, which may include the arrangement of a Bucky diaphragm 37 between the body being pictured and the cassette enclosed film.

In thus applying X-rays to the bodies of patients for either therapeutic or radiographic purposes, it is highly desirable to determine accurately the total amount or quanta of X-rays applied to the body during the exposure interval, and to terminate the exposure precisely as and when a predetermined or selected X-ray quanta has been applied. Such accuracy is highly desirable for radiographic purposes in the interests of obtaining exposed films of optimum density. The accurate determination of ray quanta during X-ray therapy is likewise of importance, especially where the desired exposure is near the safe exposure limit for the subject being treated.

To this end, X-ray sensitive means may be disposed in any convenient location in the path of the X-ray beam 17. As shown, the sensitive means may be mounted upon the side of the body 33 remote from the X-ray source, so that rays, in reaching the sensitive means, will first pass through the body 33. Any suitable ray sensitive means may, of course, be employed for ray detecting purposes. To that end, the sensitive means may embody a detector element or cell 38 which may comprise crystalline, ray sensitive semi-conductor material, such as cadmium or mercury sulphide 'or cadmium selenide, of the sort disclosed in an application for United States Letters Patent Serial No. 250,141, filed October 6, 1951, on the invention of John E. Jacobs in Interval Timing Apparatus, which issued on May 22, 1956 as US. Patent No. 2,747,104. Such crystalline semi-conductor materials have impedance characteristics which vary precisely in proportion to the quanta of X-rays applied thereto, and, consequently, a unit 38 comprising such ray sensitive materials may be employed in a suitable electrical integrating system to precisely measure the X-ray quanta applied through the body 33 and upon the sensitive detector element 38.

Alternately,'the element 38 may comprise a photosensitive cell of the sort disclosed in United States Letters Patent No. 2,401,289 of May 28, 1946, covering a Photoelectric Timer System invented by Russel H. Morgan and Paul C. Hodges, such photoelectric unit serving to measure X-ray quanta applied upon a sheet of ray sensitive fluorescent material 38 in terms of the amount of light emitted by the fluorescent sheet under the influence of X--rays impinging thereon.

The element 38, whether the same be directly ray sensitive crystalline material, such as cadmium sulphide, or whether the same be a photo cell actuated by light rays emitted by an X-ray sensitive fluorescent screen 38, may be electrically connected in an electrical translation system 41 for the operation of the switch 30, in order to open the same at the conclusion of a measured exposure interval. As shown, the translation system 41 may comprise means for inte rating the response of the detector unit 38, to thereby measure total X-ray quanta delivered during an exposure interval being measured, and for actuating a load device 42 for the disablement of the X-ray source 11 at the conclusion of the exposure interval. To these ends, the translation system 41' may embody a thyratron tube 43, the same comprising a gas filled electron flow device having a cathode 44, an anode 45,

and a control grid 46. The cathode and anode 44 and 45 may be interconnected in an output circuit including a suitable power source 47 and the load device 42 which, in the illustrated embodiment, comprises the operating coil 48 of a normally closed relay switch 49.

The control grid 46 of the thyratron may be connected in a control circuit in which the sensitive element 38 is also operatively connected; and means is provided for electrically energizing the grid 46 for the control of the tube 43 in accordance with total current caused to flow in the element 38 during an X-ray exposure interval to be timed. The sensitive element 38, accordingly, may be electrically connected with the grid 46 of the thyratron through a conductor 40. The grid control circuit may also include a preferably uni-directional power source 50 and a variable resistor 51 interconnected in series with the power source and with the sensitive element 38 through a conductor 39. The control circuit also includes an integrating condenser 52 connected between the grid of the thyratron 43 and a suitable source 53 of negative grid biasing power for the thyratron.

The control circuit may also include a normally closed disabling switch 54, interconnected in parallel across the integrating condenser 52, and a normally open anode circuit switch 58, connected between the anode 45 of the thyratron and ground in series with the power source 47 and the relay coil 48, the cathode of the thyratron being grounded, as shown, and hence connected with the grounded side of the power source 47. So long as the anode circuit switch 58 remains open, the valve 43 will continue to be inactive because its anode circuit will be open at the switch 58. Furthermore, while and so long as the switch 54 remains in closed position, a negative bias, of potential substantially in excess of that at which the thyratron fires, will be applied upon the thyratron grid '46 from the source of biasing potential 53. The condenser 52 also will remain inactive so long as the same is short-circuited by the closed switch 54, both sides of the condenser 52 being at a potential with respect to the cathode 44 equal to the negative potential applied on the grid 46.

Means is provided for opening the switch 54 coincidentally with the closure of the switch 30 for the actuation of the X-ray generator at the start of an exposure interval. Accordingly, when the switch 54 opens and switch 58 simultaneously closes, the thyratron will continue to be inactive because biased beyond cut-off. In this connection, the condenser 52 will have no charge in so far as the grid of the thyratron is concerned, but its grid connected side will be at the negative potential with respect to said cathode 44 as supplied by the power source 53. As electrical current is delivered from the detector 38 through the conductor 40 to the grid 46 of the thyratron and the grid connected side of the condenser 52, as the result of X-ray excitation of the detector 38, the grid connected side of the condenser 52 will progressively lose negative electrons, thereby becoming more and more positively charged. After an interval determined by the current flow controlled by the detector, the capacity of the condenser 52, and the bias voltage applied on the grid 46 while the switch 54 is closed, the difference of potential between the control grid and cathode of the thyratron will increase to the bias voltage level at which the thyratron may fire. When the thyratron is thus fired or placed in operation, it will energize the relay coil 48 and cause the switch '49 to open. After being triggered, the thyratron 43 will continue in operation until the switch 55 is reclosed and the switch 58 reopened.

Any preferred means may be employed for utilizing the foregoing operation of the thyratron for the control of the X-ray source 11. As shown, such control may be accomplished by providing a relay 55 having an actuating coil 56, a normally open switch 57, the normally open switch 58, and the normally closed switch 54. By energizing the coil 56, the normally closed switch 54 may be opened, and thereafter may remain open until the coil 56 is again de-energized. Conversely, the switches 57 and 58 may close when the coil 56 is energized, and thereafter may remain closed so long as said coil remains energized. The coil 56 may be connected with the power source 25 in series with, and hence under the control of a normally open control switch 59, preferably of the manually operable, push button type. The normally open switch 57 may also be connected in series with the normally closed relay switch 49 and the operating coil 31 of the normally open switch 30, to form a series circuit connected with the power source 25. The normally open switch 58 may be interconnected in the plate circuit of the thyratron, in series with the power source &7 and the operating coil 48 of the load device 42.

In order to place the X-ray generating tube 11 in operation for the application of the X-ray beam 17 to the body 33, the sensitive picturing material 35, and the detector means, the switch 59 may be closed, as by the action of the roentgenologist in operative charge of the equipment, thereby energizing the relay coil 56 to open the switch 54 and to close the switches 57 and 53. Closure of the switch 57 will complete a relay energizing circuit through the switch 49, which at such time is in closed condition, and the operating coil 31 to thereby cause closure of the normally open switch 30, in order to start the X-ray generator 11 in operation. Thereafter, the X-ray generator will remain in operation until a predetermined X-ray exposure, measured in terms of X-ray quanta, shall have been applied to the body 33. Thereupon, the thyratron tube 43 may be caused to fire as the result of the integrating action of the condenser 52. When the thyratron thus is activated, it will complete an operating circuit through the switch 58, which at such time is in closed condition, in order to energize the coil 48 and thus open the energizing circuit of the coil 31 at the switch 49. The coil 31 being thus de-energized, the switch 38 will open, thereby removing the electron driving potential from the anode and cathode of the X-ray tube, thus disabling the same as an X-ray source, whereby the application of the X-ray beam upon the body 33 will be discontinued.

It will be seen from the foregoing that the integrating condenser 52 may be operated substantially exactly for the measurement of X-ray quanta applied to the body 33 during an exposure in order to fire the thyratron precisely when the body has received a preselected amount of X-ray irradiation. It will be obvious, also, that opera tion of the X-ray generator 11 will be discontinued as the result of the operation of the thyratron after a short time interval required for the successive operation of the switches 49 and 3t). Ordinarily, the operation of such relay switch means requires an interval of the order of one-fiftieth second. Such delayed switch operation, accordingly, is a factor which, if disregarded, will introduce errors in the determination of X-ray exposure intervals, such errors becoming proportionally greater as the measured exposure interval becomes smaller.

In order to compensate for operational lag of the control relay means, the present invention provides for anticipating the termination of the exposure interval and for firing the thyratron sufficiently in advance of the end of the exposure interval to cause discontinuation of the operation of the X-ray source 11 precisely at the end of the exposure interval as measured by the condenser 52. The operation contemplated by the present invention is obtained by causing the bias voltage delivered from the source 53 to vary, during the course of the exposure interval being measured, whereby to .afiect the charging range of the condenser 52, so that, regardless of the rate of fiow of condenser charging current or the duration of the interval being measured, the thyratron will be caused to operate sufficiently in advance of the end of the 7 measured interval to compensate for the operational lag of relay switching means driven by the thyratron.

The charging characteristics of the integrating condenser 52, where the power source 53 supplies a bias potential of constant value, are shown in Fig. 2, in terms of elapsed time, between Zero charge when both sides of the condenser are at the voltage supplied from the source 53, upon the opening of switch 54, and fully charged condition when the thyratron grid connected side of the condenser reaches the critical thyratron actuating voltage, the charging characteristics of the condenser being shown for various values of charging current supplied through the conductor 40 under the control of the sensitive detector 38. The charging current thus supplied to the condenser 52 will, of course, vary with the quality or intensity of the X-rays comprising the beam 17, the distance between the ray source 11 and the detector means, the thickness and texture of the object 33, and the ray absorbing quality of its constituent material.

The several lines 1 -1 shown in Fig. 2 of the drawings, represent the charging characteristics of the condenser 52 in response to emitted rays of various intensity values. Line I represents the action of the condenser when rays are emitted at relatively high intensity such that the detector 38 releases condenser charging current at a rate such that the condenser is charged to thyratron firing condition during a single X-ray pulse interval. The lines 1 -1 show condenser action where emitted ray intensities are such that the condenser is charged at rates respectively requiring the delivery of two-ten ray pulses to charge the condenser to thyratron firing condition. It will be noted that the lines I 4 each include one or more portions representing inter-pulse intervals during which rays are suppressed and the condenser consequently is not charged.

The operational lag time of the relay switches and 49 may be empirically determined for any particular installation, the same ordinarily being of the order of 0.010.02 second, where a pair of sequentially operating devices are involved. By measuring from the point on each of the lines I I which represents thyratron firing condition, that is to say, when the condenser is fully charged, a graphical distance corresponding with, say, 0.01 second, a series of points P P may be determined, such points representing the voltage transitorily applied on the thyratron grid 0.01 second before the condenser 52 becomes charged to the normal firing potential. In like fashion, by measuring a graphical distance corresponding with 0.009 second from the normal thyratron firing positions on the lines 1 -1 a series of points P P can be determined. In fact, points representing any empirically determined lag time interval may be picked off on the lines 1 1 In a thyratron valve operated relay system embodying relays having operational lag time of 0.009 second, the lag interval would be compensated if the thyratron were caused to fire at the voltages represented by the points P P-P The locus of the points P P is a function of time only. Hence, if the thyratron firing grid potential could be controlled in accordance with such locus, satis factory switch operating lag time compensation would result. It is possible to accomplish such control by suitably varying the voltage applied upon the second control grid of a four element thyratron valve. It is, however, preferable to accomplish the desired advance operation of the thyratron by continuously varying the bias voltage delivered from the source 53, upon the grid remote side of the integrating condenser 52, during the interval being measured or timed.

In order to determine the nature of such required variable bias voltage, the inter-pulse periods, during which ray emission is suppressed, may be eliminated from the graph illustrated in Fig. 2, in order to produce the graph comprising Fig. 3 showing the lines il -1 with dead intervals removed and showing as curved lines a and b the loci of the points P P and the points P P In like fashion the locus of thyratron grid voltages theo retically required to anticipate and compensate for any operational lag interval can be determined.

The variable voltage required to be applied, upon the grid remote side of the condenser 52 during the timing of an exposure interval, in order to control the thyratron in accordance with firing conditions indicated by the locus of the points P P is shown in Fig. 4 as a line V The equation for this function is b b max in which V max is the steady value of maximum voltage delivered from the source 53, T is elapsed pulse-on time, and K is the operational time lag to be compensated for by substracting the same from each exposure interval measurement, the same commonly comprising a time interval of the order of 0.01-0.02 second.

Since the condenser 52 receives charging current intermittently under the control of the detector means 38, where the ray beam 17 comprises intermittent ray pulses, the present invention contemplates the application of the bias voltage V upon the grid remote end of the condenser 52 in intermittent fashion so that the voltage V as applied to the condenser will alter only during ray pulse emission periods and will remain constant during inter-pulse intervals, such condition being illustrated, in Fig. 4, by the stepped line V the same comprising successive sections of the function V and intervening steps illustrating time intervals during which the voltage remains constant.

It will be seen that Fig. 4 also reproduces the stepped lines I and 1 which illustrate two of the several condenser charging rates shown in Fig. 2, said condenser charging rates being illustrated in terms of negative voltage at the grid connected side of the condenser. By subtracting the values of the lines I and 1 from the values of the line V combined lines I and 1 may be developed which illustrate the variant voltage, with respect to the grounded cathode of the thyratron which is made to appear at the grid connected end of the condenser, and hence upon the grid of the thyratron, as the combined result of the delivery of charging current to the condenser, under the control of the detector means 38, and the application of the variant voltage V at the grid remote side of the integrating condenser 52.

The desired variable bias voltage conditions represented by the locus of the points P -P in Fig. 3, and by the function V in Fig. 4 may be substantially produced by means of a network 60 comprising a condenser 61 and a resistor 62 in conjunction with the power source 53, switching means 63 being provided for disconnecting the condenser from the charging and biasing means during inter-ray pulse intervals when no rays are being emitted from the ray source.

The values of resistance and capacity required for the elements of the network 60 may be chosen so that the actual bias voltage delivered from the source 53 upon the thyratron grid remote side of the integrating condenser 52 will vary as closely as possible in conformity with the theoretically required voltage function V shown in Fig. 4. In that conneciton, the equation of an exponential curve of simplest form and embodying resistance and capacity values is where R and C, respectively, are the resistance and capacity values of the resistor 62 and the condenser 61, and V is a factor depending upon the starting point of the exponential curve. The above exponential function can be made to correspond closely with the theoretically required variable bias voltage function V as shown by the line AV in Figs. 4 and 5, by selecting appropriate values for the factors R, C, V max and V In Fig. the stepped line AV illustrates the actual voltage applied upon the thyratron grid remote side of the condenser 52 from the power source 53 in accordance with the present invention, as shown in Fig. 7 of the drawings.

It Will be seen that the function AV; does not exactly match the theoretically desired function V It is, however, possible, by properly selecting the values of R, C, V and V to obtain exact coincidence of the functions AV and V throughout substantial intervals of time within the over-all operating range of the equipment. As shown more particularly in Fig. 4, exact c0- incidence occurs within the elapsed time range from 0.0125 to 0.025 second. It is, however, possible to select values of R, C, V and V max which will afiord an exact match of the functions V and AV in any desired zonal position within the elapsed time measuring range of the apparatus.

As shown in Fig. 7, the power source 53 may comprise a full wave rectifying system 64 embodying an electron flow device 65 and a transformer 66 having a center tapped secondary winding and a primary winding interconnected in parallel with the winding 20 of the transformer 19 and hence powered from the same power source which energizes the X-ray generator. Full wave rectified power thus may be delivered through a filtering network comprising interconnected condensers 67 and 68, resistors 69 and 70 and an adjustable potentiometer 71, between ground and a supply terminal 72, whence maximum bias potential may be delivered through the switch means 63 and the resistor 62 to the grid remote side of the condenser 52. Bias potential may'also be delivered from the supply terminal 72 through a resistor 73 to one side of a ballast resistance 74, the other side of which is grounded, whereby to provide a secondary bias supply terminal 75 adapted to deliver bias potential at a level substantially lower than that at which bias potential is supplied from the terminal 72, said auxiliary supply terminal 75 being interconnected with the grid remote side of the condenser 52 through a normally closed switch 76 operable with the short circuiting switch 54.

It will be seen from the foregoing that, when the equipment is in stand-by condition, the switches 54 and 76 being closed, a negative bias voltage will be applied from the supply terminal 75 through the normally closed switches 54 and 76 to the grid of the thyratron valve and to both of the opposite sides of the condenser 52. Upon closure of the switch 59 to initiate an exposure interval, the switches 54 and 76 will open to disconnect the bias supply terminal 75 from the condenser and to free the condenser for integrating action. Bias voltage of a substantially higher order, as supplied from the terminal 72, however, will be applied to the grid remote side of the condenser 52 in response to closure of the switch 63. This voltage, because of the time constant characteristics of the resistor-condenser network 60, will not immediately reach its full value upon the grid remote side of the condenser 52, but the voltage applied on said grid remote side of the condenser will follow the exponential curve of actual bias voltage as shown in Fig. 5, as the interval being timed progresses. As a consequence, the application of charging current upon the condenser 52 through the conductor 40 from the detector means 38 will result in the operation of the thyratron valve 43 in the manner desired.

It should be noted, however, that, since the charging current, as shown in Fig. 2, is delivered to the condenser 52 in successive pulses and alternate inter-pulse intervals during which no current is delivered to the condenser, it is necessary to provide means for applying the compensating bias AV in the manner indicated at AV in Fig. 5. It is also necessary to arrange to disconnect the correctional bias from the condenser during interpulse intervals in order to avoid the exponential alteration of the charged condition-of the condenser during such inter-pulse intervals and hence introducing operational inaccuracies in the system. If no provision were to be made for disconnecting the exponentially varying bias during inter-pulse intervals, voltage would be supplied to the thyratron grid in the manner indicated in dotted lines in Fig. 6 due to the continuation of the time variable biasing action on the thyratron grid remote side of the integrating condenser, during inter-pulse intervals. If the exponential lag anticipating and correcting bias potential were to be in operation continuously during the interval being measured, the voltage range, through which the timing capacitor would be charged, would be effectively increased, as indicated in Fig. 6, so that thyratron actuating voltage would not be attained until substantially more than the desired total X-ray quanta had been emitted. Accordingly, the present invention, as shown more especially in Fig. 7, provides a control system 77 for actuating the switch 63 to switch the anticipator circuit comprising the exponential biasing means into and out of operation in concert with the ray pulsations emitted during the period being measured.

To these ends the detector 38 may conveniently comprise a photoelectric tube of the sort disclosed in Letters Patent of the United States No. 2,231,697 of February 11, 1941, covering the joint invention of Vladimir K. Zworykin and Richard L.- Snyder, ]r., such tube having an anode A, a light sensitive cathode C and a plurality of dynode electrodes, D D including an end dynode D The several dynodes D D are interconnected by resistors, and the dynode D is'connected with the oathode C through a resistor. The cathode of the phototube, as by means of a conductor 78, may be connected with a suitable source 79 of relatively high voltage negative potentiaLpreferably through an adjustable resistance 80. The anode of the detector tube, by means of a conductor 81, may be connected with the translation system 77 to operate and control the same in accordance with the pulsations ofthe ray beam 17. The 'end dynode D may be connected with the grid connected side of the integrating condenser 52 through the conductor 40, while the dynode D may be connected, as by means of a conductor 82, with a source 83 of relatively low voltage negative potential.

The system 77 may comprise a series of triode electron flow valves V V V and V the valve V forming the switch means 63. The anodes of the valves V V 2 and V and of the phototube 38 are connected with a source 84 of relatively low voltage positive potential through resistors R R R and R The anode of the valve. V is connected with the interconnected sides of the condensers 52 and 61 through the adjustable resistor 62. The cathodes of the valves V V V and V may be connected to ground.

The control grid of "the valve V may be connected with the anode of the phototube 38 through a condenser 85 and with ground through a resistor 86 and a one-way current flow diode device 87 connected in parallel with respect to the resistor 86. The grid of the valve V may be connected with the anode of the valve V through a resistor 88 and a condenser 89, the negative low voltage power source 83 being connected through resistors 90 and 91, in series, with the interconnected sides of the reslstance 88 and the condenser '89, a one-way current flow diode 92 being connected in parallel with the resistor 91 and the interconnected ends of the resistors 90 and 91 being connected to ground through a resistor 93.

The control grid of the valve V may be connected with the anode of the valve V through a condenser 94. The control grid of the valve V may also be connected to ground through a resistor 95 and a one-way current flow dlode 96 connected in parallel with the resistor 95. The control grid of the switch valve V may be connected with the anode of the valve V through a resistor 97 and a condenser 98. Power may be delivered, between the pulse intervals.

11 cathode of the switch valve Vgand the interconnected sides of the mid connected resistor 97 and the condenser 98, by means of a transformer 99 having a primary winding 100 energized from the power source 25. The secondary winding 101 of the transformer may supply power for energizing the cathode filaments of the valves V V V and V One end of the secondary Winding 101 may be connected through a resistor 102 with the interconnected sides of the resistance 97 and the condenser 98, a one-way current flow diode 103 being connected in parallel with the resistor 102. The other end of the winding 101 may be connected through a one-way cunrent flow diode 104 and a resistor 105 with the cathode of the switching valve V a condenser 106 being interconnected between the transformer connected ends of the resistors 102 and 105. A resistor 107 also may be connected between the transformer connected end of the resistor 102 and the transformer remote end of the resistor 105.

When ray pulses impinge upon the detector tube 38, corresponding current pulses which pass from the end dynode to the anode of the phototube will traverse the resistor R to the source 84, thus developing corresponding voltage pulses across the resistor R Accordingly, the voltage at the source remote end of the resistor R will periodically fall below the voltage value supplied from the source 84, as shown in Fig. 8 of the drawings. This voltage fluctuation is reflected through the condenser 85 to the grid of the tube V as shown in Fig. 9 of the drawings. The valve V is in conductive condition prior to the occunrence of an X-ray pulsation and is held conductive by the diode 87 and by virtue of the fact that there is no voltage drop across the resistor 86, the grid of the valve V being thus effectively tied to its cathode so that the tube remains in conductive condition.

As the voltage established across the resistor R in response to the application of an X-ray pulsation upon the detector tube is reflected on the grid of thevalve V by the condenser 85, a voltage is developed across the resistor 86 which causes a reduction below zero value in the voltage on the grid of the valve V thereby reducing the conductivity of said valve. The magnitude of the voltage pulsation, however, is not great enough to make the tube V completely non-conducting, and it is for that reason that the tubes V and V are operated as amplifiers to increase the signal gain so that the end valve V, will become conductive at the start of an X-ray impulse and non-conductive at the conclusion of a ray pulsation. Figs. 10, 11 and 12 respectively illustrate the operation of the valves V V and V It will be seen that while the valves V and V produce an amplification gain, they nevertheless do not become sharply conducting and nonconducting with the commencement and termination of a ray pulsation.

In this connection, the plate voltage pattern developed in the valve V is fed through the condenser 89 upon the grid of the valve V Accordingly, when the X-ray pulse is a maximum, the valve V is in its maximum conducting condition, the voltage at the anode of the valve V being a minimum when the X-ray pulse is at its maximum value. Such minimum voltage is then applied through the condenser 94 to the grid of the valve V in order to decrease the conductivity thereof when the X- ray impulse is at maximum value. Accordingly the voltage developed at the anode of the valve V is a maximum when the ray impulse is at its maximum value. The anode voltage thus developed in the valve V is applied through the condenser 98 upon the grid of the switching valve V to render the same conductive during X-ray pulse intervals and non-conducting during interpulse intervals.

It will be understood from the foregoing that the valve V should be relatively non-conductive during ray inter- Its grid voltage consequently must be negative with respect to its cathode during such intervals. The necessary negative voltage may be derived from the source 83 through the voltage divider comprlsing the resistors 90, 91 and 93 and the diode 92. The diode 92 as well as the other diodes 87, 96 and 103 serve as direct current restorers, that is to say, they permit the voltage change, which occurs when the pulse enters and leaves, to have a full etfect upon the grid of the valve with which connected, and they prevent the same from assuming opposite polarity due to the one-way conductivity of the diode.

In order to assure the negativity of grid voltage on the valve V and the non-conducting condition thereof during inter-pulse intervals, its grid is connected through the resistors 97 and 102 and the diode 103 to the point of connection of the resistor 102 and its diode 103 with the condenser 106 and the resistor 107. This connection point is negative with respect to the cathode of the valve V by the voltage which is developed across the resistor 107 through the operation of the power input circuit means comprising the transformer 99 and the diode rectifier 104, power delivered through the transformer being rectified by the diode 104 and applied to the condenser 106 and network of resistors and 107 which operate as a voltage divider.

Where the ray detecting means comprises a sensitive panel, screen or layer of fluorescent material 38 for the operation of a phototube, the possibility exists of introducing errors in the measurement of elapsed time intervals due to slow image decay time constant characteristics of the fluorescent material constituting the element 38. Ordinary fluorescent materials have image decay time constant characteristics such that the same may be employed for measuring intervals of duration of the order of 0.1 second or more, but have image persistence of such duration after the extinction or termination of exciting rays as to cause erroneous operation of an integrating system in which employed, where the interval being measured is short.

Due to image persisting characteristics of fluorescent material, excitation thereof and consequent light production and emission by the element 38 may result in the application of light upon the detector tube 38 during the initial portions of inter-pulse intervals. The relatively slow decay time constant of ordinary fluorescent materials thus will result in the delivery of more current from the detector tube 88 than would occur if light emission from the material of the element 38' were to be precisely re sponsive to X-ray incidence thereon. Where the interval being measured comprises an extended period of suflicient length to include several successive ray pulsations, the integrating errors caused by image persistence can be compensated for by adjusting the bias supplied upon the thyratron from the power source 53, since the error, measured over several pulsations tends to become directly proportional to the length of the measured interval.

Where, however, the exposure interval is of duration of the order of less than 0.1 second, equivalent to less than twelve ray pulsations, where the rays are produced in a tube energized by full wave rectified sixty cycle alternating current power, the error introduced as the result of slow image decay characteristics in the element 38 becomes appreciable. The error, furthermore, increases as the exposure time interval diminishes, the extent of error being somewhat erratic and dependent to some extent upon the relative position of the instant of exposure interval termination in or with respect to the ray source energizing power cycle. Accordingly, where the ray sensitive unit is to be used in conjunction with equipment for measuring exposure intervals of duration of the order of 0.1 second or less, and particularly for ultra short exposure intervals of the order of one-thirtieth second, and less, the element 38' should comprise ray sensitive fluorescent material, such as calcium tungstate, having substantially instantaneous image decay characteristics, that is to say, material in which ray induced luminosity becomes extinguished as during an interval of the order of one-thousandth of a second, after the termination of ray impingement thereon.

Where such sensitive screen elements having high speed image decay characteristics, as differentiated from standard or slow speed screens, are used, it is desirable to compensate the interval measuring equipment for changes in the anode-cathode voltage at which the X-ray generating tube is operated. It is to this end that the transformer 66 is energized by power derived from the adjustable secondary winding 21 of the transformer which supplies energy at desired adjustable voltage for application between the anode and cathode of the generator tube.

The output of the transformer 66 is subjected to full wave rectification and applied to the filter network comprising the condensers 67 and 68 and the resistor 69. The voltage thus produced across the condenser 68 is applied across the resistor 70 and potentiometer 71, the positive end of the resistor 70 being grounded so that the voltage derived from the adjustable potentiometer 71 is negative with respect to ground. This voltage is used as the full bias voltage, that is to say, the voltage labeled 100 percent in the graphical illustrations comprising Figs. 2-6. This full bias voltage is connected to ground across resistors 73 and 74, which are preferably of identical value so that their interconnected ends at the terminal '75 provide a 50 percent voltage value as the bias voltage initially applied upon the thyratron grid at the commencement of interval measurement. It will be obvious, of course, that an adjustable potentiometer could be substituted for the resistors 73 and 74 in order to permit adjustment of the bias voltage obtainable at the delivery terminal 75.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of demonstrating the invention.

The invention is hereby claimed as follows:

1. Apparatus for actuating operable devices at the conclusion of a measured time interval comprising a gaseous conduction electron fiow valve having an anode, a cathode and a control grid, an integrating condenser connected on one side with said grid, a switch for initially biasing said grid and both sides of said condenser at selected negative voltage values with respect to the cathode of the flow valve to hold the same normally in non-conducting, inoperative condition, means for applying current pulses during successive intervals to charge the condenser and thus decrease the negativity of the voltage applied on the grid in accordance with the charging rate of said condenser, a circuit embodying a condenser operable to provide a progressively varying bias voltage, and a switch operable in synchronism with said current pulses for applying said bias voltage of progressively increasing negativity upon the grid remote side of the condenser during said pulse intervals only, to alter the condenser charging rate in accordance with said bias voltage for the operation of said operable devices in advance of the end of the interval being measured to thereby compensate for the operational lag interval of said devices.

2. Control apparatus comprising a gaseous conduction valve having a control grid, an integrating condenser connected with said control grid, means to apply successive current pulsations upon the condenser to charge the same progressively and thus to alter the voltage applied upon said grid, a circuit including a condenser operable to provide a bias voltage of progressively changing value, and intermittently operable switch means for applying said progressively varying bias voltage upon the grid remote 14 side of said condenser to thereby alter the condenser charging rate in accordance with said bias voltage.

3. Control apparatus comprising a gaseous conduction valve having a control grid, an integrating condenser connected with said control grid, means to apply successive current pulsations upon the condenser to charge the same progressively and thus to alter the voltage applied upon said grid, a circuit including a condenser operable to provide a bias voltage of progressively changing value, an electron flow switching Valve operable in synchronism with the pulsations of charging current for applying said progressively varying bias voltage upon the grid remote side of said condenser to thereby alter the condenser charging rate in accordance with said bias voltage.

4. Control apparatus comprising the combination with a penertating ray source, means including a control switch for controlling the emission of ray energy pulsations from said source, and a ray responsive detector, of a gaseous conduction valve having a control grid, an integrating condenser connected with said control grid, means operable under the control of said detector for producing successive current pulsations, corresponding with ray quanta emitted by said source, and for applying such pulsations upon the condenser to charge the same progressively and thus to alter the voltage applied upon said grid progressively in accordance with emitted ray quanta, a circuit including a condenser operable to pro vide a bias voltage of progressively changing value, and an electron flow switch device operable in synchronism with said current pulsations for applying said progressively varying bias voltage upon the grid remote side of said condenser, at intervals corresponding with said current pulsations, to thereby alter the condenser charging rate in accordance with said bias voltage.

5. Control apparatus comprising the combination with a penetrating ray source, means including a control switch for controlling the emission of ray energy pulsations from said source, and a ray responsive detector, of a gaseous conduction valve having a control grid, an integrating condenser connected with said control grid, means operable under the control of said detector for producing successive current pulsations, corresponding with ray quanta emitted by said source, and for applying such pulsations upon the condenser to charge the same progressively and thus to alter the voltage applied upon said grid progressively in accordance with emitted ray quanta, an electron flow switch, amplifier means driven in synchronism with said pulsation producing means, under the control of said ray detector, for applying a progressively varying bias voltage upon the grid remote side of said condenser, at intervals corresponding with said current pulsations, to thereby alter the condenser charging rate in accordance with said bias voltage, and means, operable by said gaseous conduction valve, to actuate said control switch after emission of predetermined ray quanta from said source, including means for varying said bias voltage with and in proportion to variations in the average intensity of emitted rays.

6. In a system for generating X-rays of pulsating in tensity, apparatus for limiting an exposure to a predetermined quanta of X-ray energy, said apparatus comprising a ray sensitive device positioned to receive X-rays from said generating system, a biasing circuit for producing a varying bias voltage, an integrating condenser connected between the ray sensitive device and the biasing circuit, a relay switch connected across said integrating condenser for establishing an initial condition wherein said integrating condenser has no charge and wherein an initial bias voltage is applied thereto, said integrating condenser being operable to accumulate charge from the ray sensitive device in accordance with the X-ray intensity, a gaseous conduction device having a control grid coupled to the integrating condenser, said gaseous conduction device being responsive to the appearance of a grid voltage upon its control grid, said grid voltage being an addition 15 of the varying bias voltage and a voltage appearingtacross the integrating condenser corresponding to the charge accumulated from the ray sensitive device, said gaseous conduction device being operable to cause relay action to terminate the X-ray exposure.

0 7. The apparatus according to claim 6 wherein the circuit for producing the bias voltage is coupled to a switching circuit for disabling the biasing circuit and holding the bias voltage constant during intervals between the pulsations of the X-rays, and thereby allowing the bias voltage to vary only during times of ray impingement upon the ray sensitive device.

8. The apparatus according to claim 6 wherein the biasing circuit comprises a condenser resistively coupled across a source of unidirectional voltage by an electron tube, said electron tube being conductive during intervals of X-ray impingement upon said ray sensitive device and being nonconductive between the intervals of X-ray impingement, thereby allowing the condenser to charge only during the intervals of ray impingement.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,289 Morgan et al. May 28, 1946 2,438,303 Simmon Mar. 23, 1948 2,453,486 Ball Nov. 9, 1948 2,494,218 Weisglass Jan. 10, 1950 2,679,598 Wright et al. May 25, 1954 2,747,104 Jacobs May 22, 1956 FOREIGN PATENTS 904,916 Germany July 8, 1949 

